The use of volumetric image rendering techniques in ultrasonic scanning is becoming one of the most exciting modalities in modern medical diagnostics. Currently, a number of sophisticated systems are capable of 3D imaging and surface representation that provide patients and users with both figurative images and comprehensive data information. The diagnostic results can be displayed either as surface images (3D) or as 2D scanning representations (i.e., a conventional representation) in order to give better detail.
Among the most important advantages of 3D systems is the capability provided thereby of scanning a volume of the organ for imaging purposes and then storing the entire information obtained in a manner so as to be able to display the desired scanning plane by simply positioning a cursor on the surface of the related organ. This capability allows the diagnosis to be reduced to a single-pass scanning action in obtaining all of the useful information.
To provide imaging system with 3D scanning capability, several probe technologies have been developed and are commercially available today. Generally speaking, ultrasonic scanning probes having a 3D capability currently belong to three construction sub-groups: matrix arrays or fully electronic probes (wherein all scanning is done by electronic phasing), moving phased arrays or hybrid probes (wherein one scanning operation is carried out by electronic phasing while a second is performed mechanically using a motor drive) and 2D mechanically moving transducers or fully mechanical probes (wherein mechanical scanning in both directions is provided).
The matrix array approach uses arrayed transducers having vibrating elements disposed in two crossing directions so as to form an emission surface. The vibrating element sizing of these transducers should be on the order of the transducer wavelength, and thus this approach requires a very large number of vibrators (typically more than 3000) that must be addressed.
Moving phased arrays are more commonly used because of their relative simplicity of construction and the possibility of using existing imaging systems (for software updating and motor control).
Finally, 2D mechanical moving transducer devices are much less commonly commercially available because of the intricate moving mechanism employed and the associated effective lack of reliability.
Presently, most 3D ultrasound imaging systems or 3D upgradeable systems are equipped with hybrid probes wherein an electronic array transducer is swung, tilted or rotated to enable impinging of a volume sonification. Transducers such as phased-arrays, linear arrays (straight and curved) and 1.5D arrays are suitable for this purpose. Commonly, the transducer is mounted in a coupling liquid bath which is formed by the probe housing. A motor is then coupled to the transducer carrier and the front shell of probe is made up of acoustically transparent material so as to not disturb sound propagation path. However, solid state materials that perfectly match the acoustic impedance of the human body and the coupling liquid do not exist in practice so that artifacts are usually encountered on the resultant images and these contribute to degrade the quality of system. Different potential solutions and technical approaches have been attempted to improve the coupling problem between the array transducer and the probe shell and these efforts have been more or less successful. However, the general problem is even more complicated with systems employing curved linear array transducers.
Considering some examples of the related prior art, U.S. Pat. No. 5,159,931 to Pini discloses an ultrasonic apparatus which enables three dimensional reconstruction of anatomical structures. According to this patent, the volume is obtained either by rotating a sector scan transducer of an angle of 180 degrees or by using a matrix array. The patent relates generally to an apparatus including a phased array transducer which is rotated by means provided for this purpose. Thus, a principle is stated with some limitations, but no detail is provided on how the transducer is acoustically coupled to the probe shell.
In U.S. Published Patent Application No. 2003/0055338 to Steininger et al, an ultrasonic probe for endosonography is disclosed which includes a transducer array pivotally mounted on the probe housing and coupled to means for providing rotation thereof around an axis of rotation. The transducer array is constructed in such a manner as to be able to swing through a sector of angle underneath the probe front shell. This method allows the probe to scan regions of an organ that are normally not seen by probes having a fixed transducer. A stepper motor is coupled to the transducer to provide the apparatus with accurate positioning and movement. A coupling fluid is used to fill the space separating the transducer as well as the interior surface of the probe shell. Unfortunately, in practice, the coupling fluid volume located between the transducer and the probe front shell is responsible for acoustic artifacts which are visible in the proximal zone of the resultant images.
The basic concept disclosed in the Steininger et al patent is similar to that disclosed in U.S. Pat. No. 5,152,294 to Mochizuki et al wherein a three dimensional ultrasonic scanner of hybrid type is described. An array transducer of a curved shape is provided internally. A coupling fluid bath is disposed between the transducer and the probe front shell. A volume of liquid is limited by a flexible membrane sealed to the edges of the transducer and the periphery of the front shell. This construction prevents the drive means (motor) from being exposed to liquid immersion. In a manner designed to reduce acoustic artifacts from being formed through the coupling bath as described above, the longitudinal radius of curvature of the transducer is made smaller than that of the corresponding internal surface of probe front shell so as to provide oblique reflection of acoustic energy emitted from, and received by, the transducer. Further, an absorbing material is disposed at the extremities of the front shell to cancel ultrasonic waves hitting this area, so no return echoes are produced. However, this method of reflecting echo suppression results in a complicated probe construction and the acoustic path for every single element of array is inherently modified, from the center to the outermost elements of the array, thereby creating an undesired apodisation function. Another shortcoming of this approach concerns the liquid bath surrounding the transducer; strong liquid turbulences will occur when the movement of the transducer is accelerated, thereby resulting in increased resistance to transducer movement and in consequent higher energy consumption by the drive means.
U.S. Pat. No. 6,213,948 to Barthe et al describes a three dimensional ultrasound imaging probe wherein an array transducer swings over a concentric radius with the probe front shell so as to continuously maintain the same distance between all of the transducer elements and the shell. The volume of coupling fluid is also controlled so as to be a minimum, to thereby ensure that the size of the probe is compatible with clinical use.
Although the Barthe et al patent discloses an interesting approach to maintaining the liquid bath at a constant distance with respect to the probe shell over the entire surface of transducer, this approach is only suitable for flat linear or flat phased arrays wherein the transducer can be manufactured with an elevational front curvature equal to that of the probe front shell. As disclosed in the Barthe et al patent, the transducer elevational section is at a constant spacing from the shell, thereby optimizing the acoustic path thereof. Unfortunately, an extension of this approach to probes using curved linear array transducers which are based on swinging or tilting of the transducer is inhibited or prevented by the fact that while the array transducer still exhibits same elevational radius of curvature over the azimuth length thereof, the internal surface of the probe shell will have the radius of curvature thereof changed from the center (the largest change) to the edges (the smallest) as the intersection of two curved surfaces, one along the azimuth and the other along the elevation. In fact, for curved array based hybrid probes, the more the transducer azimuth axis is curved, the higher the difference in the radius of curvature of the shell. Therefore, in some regions of the transducer front face, the lens surface, or the external surface of transducer will not be parallel to the internal surface of shell. As a consequence, the transmitted ultrasonic waves are inherently bent at a deviation angle according to the difference in refraction index between the lens material and the coupling liquid. This phenomenon is even more marked with temperature change, where the refraction index of the liquid will vary much more than that of the corresponding solid materials such as those of the lens or probe shell. Generally speaking, if no aberration correction is provided by the imaging system, as is usually the case, a degradation of the image quality can be observed at the sides of images.
In view of all of the above shortcomings and drawbacks of prior art three dimensional probes and, more particularly, of curved linear array transducer-based swinging probes (hybrid devices), there is obviously a need for improved three dimensional hybrid probe constructions wherein acoustic artifacts, due to the coupling fluid, are avoided and higher tilting rates and acceleration of the transducer in the probe housing are permitted.